A piezoelectric transformer (solid former) is configured to input low voltage and output high voltage by utilizing a resonance phenomenon of a piezoelectric vibrator (see, for example, Patent Document 1). A characteristic of a piezoelectric transformer is that the energy density of a piezoelectric vibrator is higher comparing with an electromagnetic type. Therefore, the size can be reduced, whereby piezoelectric transformers are used for cold-cathode tube lighting, liquid crystal backlight lighting, a small AC adapter, a small high-voltage power source, and the like.
FIG. 8 shows a piezoelectric transformer, in which FIG. 8[1] is a perspective view, FIG. 8[2] is a side view, and FIG. 8[3] is an equivalent circuit diagram. Hereinafter, explanation will be given based on these Figs.
A piezoelectric transformer 10 includes primary electrodes 12 and 13 and a secondary electrode 14 on a piezoelectric vibrator 11, and the primary side is polarized in a thickness direction (arrow 15) and the secondary side is polarized in a length direction (arrow 16). The primary electrodes 12 and 13 are opposite each other sandwiching the piezoelectric vibrator 11. The piezoelectric vibrator 11 is in a plate shape (rectangular parallelepiped shape) having the length L, the width W and the thickness t. In the length direction of the piezoelectric vibrator 11, the primary electrodes 12 and 13 are provided from one end to L/2 in a width direction, and the secondary electrode 14 is provided in a thickness direction on the other end. When the voltage of a natural resonance frequency fr determined by the length dimension is inputted into the primary side of the piezoelectric transformer, intense mechanical vibration is caused in the piezoelectric transformer due to the inverse piezoelectric effect of the piezoelectric transformer, and with the piezoelectric effect thereof, an intense voltage Vo appropriate to the vibration is outputted to the secondary side.
The distribution of displacement and stress at the time of driving the piezoelectric transformer 10 is shown in FIG. 8[2]. Parts holding the piezoelectric vibrator 11 are nodes located at positions which are ¼ of the length from both ends in the case of λ mode. An equivalent circuit in the vicinity of the resonance frequency fr of the piezoelectric transformer 10 can be written as shown in FIG. 8[3].
[Patent Document 1]
Japanese Patent Application Laid-open No. 8-32134